Micromechanical component and method of manufacturing a micromechanical component

ABSTRACT

A micromechanical component and method for its manufacture, in particular an acceleration sensor or a rotational speed sensor, includes: function components suspended movably above a substrate; a first insulation layer provided above the substrate; a first micromechanical function layer including conductor regions provided above the first insulation layer; a second insulation layer provided above the conductor regions and above the first insulation layer; a third insulation layer provided above the second insulation layer; a second micromechanical function layer including first and second trenches provided above the third insulation layer, the second trenches extending to the third insulation layer above the conductor regions and the first trenches extending to a cavity beneath the movably suspended function components in the second micromechanical function layer.

FIELD OF THE INVENTION

[0001] The present invention relates to a micromechanical component, inparticular an acceleration sensor or a rotational speed sensor includingfunction components suspended movably above a substrate, and acorresponding manufacturing method for the micromechanical component.

BACKGROUND INFORMATION

[0002] Although it may be applied to any micromechanical components andstructures, in particular to sensors and actuators, the presentinvention and the underlying problem are elucidated with reference to amicromechanical acceleration sensor that may be manufactured usingsilicon surface micromachining technology.

[0003] Acceleration sensors, in particular micromechanical accelerationsensors manufactured using surface or volume micromachining technology,have an increasing market share in the automotive equipment industry andare increasingly replacing the piezoelectric acceleration sensorscustomarily used.

[0004] Conventional micromechanical acceleration sensors normallyoperate so that a flexibly mounted seismic mass device, which isdeflectable in at least one direction by an external acceleration, ondeflection causes a change in the capacitance of a differentialcapacitor device that is connected to it. This change in capacitance isa measure of the acceleration.

[0005] German Published Patent Application No. 195 37 814 describes amethod of manufacturing surface micromechanical sensors.

[0006] A first insulation layer of a thermal oxide (approximately 2.5 μmthick) is first deposited on a silicon substrate. Then, a thin(approximately 0.5 μm thick) polysilicon layer is deposited on thisinsulation layer. The polysilicon layer is subsequently doped from thegas phase (POCl₃) and structured by a photolithographic process. Thisconducting polysilicon layer to be buried is thus subdivided intoindividual regions that are insulated from one another and function asconductors or as surface electrodes arranged vertically.

[0007] A second insulation layer is deposited above the layers appliedpreviously. This insulation layer is made up of an oxide generated fromthe gas phase. In a photolithographic process, the top insulation layeris structured by introducing contact holes into the top insulationlayer, through which the underlying polysilicon layer may be contacted.

[0008] Then, a thin polysilicon layer, which functions as a seed forsubsequent deposition of silicon, is applied. This is followed inanother step by deposition, planarization and doping of a thickpolycrystalline silicon layer. The deposition occurs in an epitaxialreactor. Then, a structured metal layer is applied to the thick siliconlayer.

[0009] The thick silicon layer is structured in anotherphotolithographic process, in which a photoresist mask is applied to thetop side of the layer to protect the metal layer in the subsequentetching. Then, plasma etching of the thick silicon layer is performedthrough openings in the photoresist mask according to the methoddescribed in German Published Patent Application No. 42 410 45, in whichtrenches having a high aspect ratio are produced in the thick siliconlayer. These trenches extend from the top side of the thick siliconlayer to the second insulation layer. The layer is thus subdivided intoindividual regions that are insulated from one another, unless they areinterconnected by the buried conducting layer.

[0010] The two sacrificial layers are then removed through the trenchesin the area of the freely movable structures of the sensor. The oxidelayers are removed by a vapor etching method using media containinghydrofluoric acid according to a method described in German PublishedPatent Application No. 43 172 74 and German Published Patent ApplicationNo. 197 04 454.

[0011] However, there are disadvantages with the removal of thesacrificial layer by the hydrofluoric acid vapor etching method. Withthis etching method, it is very difficult to achieve a definedundercutting, i.e., the oxide is removed not only beneath the functionalor freely movable sensor structures, but also above and beneath theburied polysilicon conductors. This requires very wide conductorsbecause the possibility of lateral undercutting must be assumed. Due tothe undercutting, no conductors are allowed to pass beneath thefunctional structure. Another disadvantage is corrosion of the metallayer due to the hydrofluoric acid vapors.

[0012] If the water content of the gas phase is too high, there may besticking problems, i.e., the freely movable sensor elements may adhereto the substrate. Due to the limited oxide thickness (due to thedeposition method) of the insulation layers, the distance between thefunctional structure and the substrate is also limited.

[0013] Since the hydrofluoric acid vapor etching method is notcompatible with the materials used in CMOS technology, there may not beany integration of the sensor element and the analyzer circuit.

SUMMARY

[0014] The micromechanical component according to an example embodimentof the present invention and a corresponding manufacturing method mayhave the advantage that both the buried conductors and the sacrificiallayer beneath the freely movable structures may be made of the samelayer. Therefore, fewer layers and photolithographic processes may beneeded.

[0015] The present invention provides a layer structure and acorresponding method for manufacturing micromechanical components, e.g.,acceleration sensors having a lateral sensitivity, sacrificial layerregions being made of the same material, e.g., polysilicon, as theburied conductor regions. A defined etching of the sacrificialpolysilicon layer regions is achieved with the method according to thepresent invention, thus preventing undercutting of the buried conductorregions.

[0016] The method according to the present invention allows a simplemethod of manufacturing a sensor element using only method steps thatare conventional in semiconductor technology. In addition, only a fewlayers and photolithography steps are necessary with the methodaccording to the present invention.

[0017] The first micromechanical function layer and the secondmicromechanical function layer may include polysilicon layers.

[0018] According to another example embodiment of the present invention,the first through fourth insulation layers are oxide layers.

[0019] In removing the sacrificial layer, if the first micromechanicalfunction layer is made of polysilicon and the insulation layers areoxide layers, then etching media based on fluorine compounds (e.g.,XeF₂, ClF₃, BrF₃, etc.) may be used. The etching media have a very highselectivity with respect to silicon dioxide, aluminum and photoresist.Due to this high selectivity, the polysilicon conductor regions that arenot to be etched, in contrast with the sacrificial polysilicon layerregions, are sheathed with silicon dioxide. This prevents etching orundercutting of the polysilicon conductor regions.

[0020] This also permits a conductor to be guided beneath the freelymovable structures. Since the buried polysilicon conductor regions areno longer being undercut, they may be narrower. Reproducible removal ofthe sacrificial polysilicon layer regions that is well-defined bothlaterally and vertically is possible with the sacrificial polysiliconlayer technology described above. Due to the high selectivity of theetching medium with respect to silicon dioxide, it may be possible toimplement a multilayer system of polysilicon conductors and insulationlayers, and crossing of lines may also be possible. Large lateralundercutting widths may be feasible in sacrificial layer etching, sothat the number of etching holes in the seismic mass may be reduced oromitted entirely. This yields an increase in the seismic mass.

[0021] Since the etching process for removing the sacrificialpolysilicon layer occurs in the gas phase, no problems may exist withregard to corrosion and sticking. The silicon sacrificial layertechnology may be compatible with materials used in CMOS technology,thus permitting integration of the sensor element and analyzer circuit.

[0022] It may be possible to structure the insulation layers by dryetching processes through the choice of layer thicknesses and sequence,thus eliminating wet etching processes and yielding improved processtolerances. The distance between the freely movable structure and thesilicon substrate layer may be adjustable as needed through thethickness of the polysilicon layer.

[0023] According to another example embodiment of the present invention,the conductor regions and the sacrificial layer regions are providedthrough local implantation and subsequent photolithographic structuring.

[0024] According to yet another example embodiment of the presentinvention, contact holes for connecting the second micromechanicalfunction layer to the conductor regions are provided in the second andthird insulation layers.

[0025] According to even another example embodiment of the presentinvention, contact holes for connecting the second micromechanicalfunction layer to the substrate are provided in the first, second andthird insulation layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGS. 1 to 11 are schematic cross-sectional views illustratingthe manufacturing process for an acceleration sensor according to anexample embodiment of the present invention.

DETAILED DESCRIPTION

[0027] In the Figures, identical symbols denote identical orfunctionally equivalent components.

[0028] FIGS. 1 to 11 are schematic cross-sectional views illustratingthe manufacturing process for an acceleration sensor according to anexample embodiment of the present invention.

[0029]FIG. 1 illustrates a silicon substrate 1 to which are applied afirst insulation layer 2 and then a polysilicon layer 3 on top of thefirst insulation layer 2. Conventional deposition processes fromsemiconductor technology for depositing dielectric layers may be usedfor depositing first insulation layer 2. In addition to silicon dioxide,thus silicon nitride, dielectric layers having a lower dielectricconstant than silicon dioxide, various types of glass or other ceramiclayers may also be deposited. For the remaining description, it isassumed that first dielectric layer 2 is made of silicon dioxide formedby thermal oxidation of silicon substrate 1 and has a thickness between10 nm and 2.5 μm.

[0030] Polysilicon layer 3 has a thickness between 0.5 μm and 5 μm.After its subsequent structuring, polysilicon layer 3 yields both buriedpolysilicon conductor regions 4 and sacrificial polysilicon regions 5.

[0031] A high conductivity is required for buried polysilicon conductorregions 4, so polysilicon layer 3 is doped from the gas phase (POCl₃)over the entire surface area. Any other method of producing asufficiently highly doped polysilicon layer may also be used. If dopingof polysilicon layer 3 is desired only in polysilicon conductor regions4, the high conductivity in these regions may be produced by localimplantation, which requires an additional photolithographic process.

[0032] Then, a structuring of doped or partially doped polysilicon layer3 occurs through a photolithographic process, as illustrated in FIG. 2.This structuring of polysilicon layer 3 occurs by dry etching (plasmaetching). Polysilicon layer 3 is thus subdivided into individual,mutually insulated regions 4, 5 that function as buried polysiliconconductor regions 4 and/or as sacrificial polysilicon layer regions 5.

[0033] As illustrated in FIG. 3, a second insulation layer 6 isdeposited on the structure illustrated in FIG. 2 and structured. Thesecond insulation layer 6 in this example embodiment of the presentinvention is made of silicon dioxide, which is produced from the gasphase, e.g., by decomposition of silane. The thickness of secondinsulation layer 6 may be greater than or equal to the thickness offirst insulation layer 2.

[0034] In another photolithographic process, second insulation layer 6is structured. The oxide of second insulation layer 6 is removed inregion 7 above sacrificial polysilicon layer regions 5 and in region 8of the substrate contact. Structuring of second insulation layer 6 mayalso occur by dry etching (plasma etching).

[0035] As illustrated in FIG. 4, a third insulation layer 9 is depositedon the structure illustrated in FIG. 3. Insulation layer 9 is configuredto protect or passivate structures 25, which are later to be made freelymovable (see FIG. 11), on the lower side with respect to the etchingmedium used in sacrificial layer etching. Third insulation layer 9 maybe made of silicon dioxide produced from the gas phase, e.g., bydecomposition of silane. Insulation layer 9 is needed only in regions 7where second insulation layer 6 above sacrificial polysilicon layerregions 5 is removed.

[0036] Third insulation layer 9 may also be produced by a local thermaloxidation only in region 7. The layer thickness of third insulationlayer 9 may be between 5 nm and 500 nm.

[0037] Then, a polysilicon starting layer 10 is deposited on the surfaceof the structure illustrated in FIG. 4, as illustrated in FIG. 5.Polysilicon starting layer 10 covers the surface of third insulationlayer 9 and functions as a seed for the subsequent deposition ofpolysilicon. To deposit polysilicon starting layer 10, anyconventionally used method in semiconductor technology for deposition ofthin polysilicon layers on dielectric layers may be suitable.

[0038] In a subsequent process step, a photolithographic structuring ofpolysilicon starting layer 10 and underlying insulation layers 2, 6, 9or 6, 9 may occur by dry etching (plasma etching).

[0039] In the regions above buried polysilicon conductor regions 4,contact holes 11 are introduced into polysilicon starting layer 10 andsecond and third insulation layers 6, 9, through which underlyingpolysilicon conductor regions 4 may be contacted.

[0040] In the regions where a substrate contact hole 12 is to beproduced, polysilicon starting layer 10 and first, second and thirdinsulation layers 2, 6, 9 are structured.

[0041] As illustrated in FIG. 6, a thick silicon layer 13 is depositedin another process step. This deposition occurs in a conventionalepitaxial reactor. Such an epitaxial reactor is an installation fordeposition of silicon layers that is used in semiconductor technologyfor the production of single-crystal silicon layers on a single-crystalsilicon substrate. In the example embodiment according to the presentinvention, deposition in the epitaxial reactor does not occur on asingle-crystal silicon substrate, but instead occurs on polycrystallinesilicon starting layer 10, so that no single-crystal silicon layerdevelops, but instead there is a thick polycrystalline silicon layer 13.Polysilicon starting layer 10 becomes a part of thick polycrystallinesilicon layer 13 in this step of the process.

[0042] Since polycrystalline silicon layer 13 has a rough surface afterthis deposition, it is subsequently planarized. An electric connectionto buried polysilicon conductor regions 4 is established through thickpolycrystalline silicon layer 13 so that thick polycrystalline siliconlayer 13 is doped.

[0043] Then, a structured metal layer 14 is provided on the top side ofthick polycrystalline silicon layer 13. Metal layer 14 may be appliedover the entire surface, for example, and then structured.

[0044] Deposition of a silicon dioxide layer 15 from the gas phaseoccurs in a subsequent process step, e.g., by decomposition of silane asillustrated in FIG. 7. Silicon dioxide layer 15 may have a thicknessbetween 0.5 μm and 5.0 μm. The layer thickness of silicon dioxide layer15 may be greater than the layer thickness of insulation layer 9.Silicon dioxide layer 15 is structured by a subsequent photolithographicprocess. The structuring of silicon dioxide layer 15 may also occur by adry etching process (plasma etching). Silicon dioxide layer 15 functionsas a mask for the subsequent etching process for structuring thickpolycrystalline silicon layer 13. It also provides protection for metallayer 14 during subsequent etching. Then, dry etching (plasma etching)of thick polycrystalline silicon layer 13 is performed through openings16 in silicon dioxide layer 15, which functions as a mask, thus formingtrenches 17, 18.

[0045] The etching process stops on reaching third insulation layer 9because it has a very high selectivity of silicon with respect tosilicon dioxide. This yields exposed regions 19, 20 at the bottom oftrenches 17, 18. Trenches 17, 18 having a high aspect ratio, i.e., alarge depth and a low lateral dimension, may be produced by theanisotropic etching process. Trenches 17, 18 extend from the surface ofthick polycrystalline silicon layer 13 to third insulation layer 9.Polycrystalline silicon layer 13 is thus subdivided into individualregions that are insulated from one another so that they are not linkedtogether by buried polysilicon conductor regions 4.

[0046] The functional structure is produced through trenches 17 whichare located above sacrificial polysilicon layer regions 5, or freelymovable structures 25 (see FIG. 11) are produced after removal ofunderlying sacrificial polysilicon layer regions 5. The connectingregions are defined and insulated by trenches 18.

[0047] As illustrated in FIG. 8, a fourth insulation layer 21 protectingor passivating the side walls of trenches 22 with respect to the etchingmedium used in sacrificial layer etching is deposited. This fourthinsulation layer 21, which functions as a side wall passivation, may beproduced from silicon dioxide deposited from the gas phase, e.g., bydecomposition of silane. Since insulation layer 21 is required only onside walls 22 of trenches 17, 18, it may also be produced by localthermal oxidation or by an oxide formed in the oxygen plasma. The layerthickness of insulation layer 21 may be between 5 nm and 500 nm.

[0048] To allow the etching medium to be introduced through trenches 17to sacrificial polysilicon layer 5 to remove sacrificial polysiliconlayer regions 5, third and fourth insulation layers 9, 21 are removed atthe bottom of trenches 19, 20. This yields trenches 17 having exposedregions 23 of sacrificial polysilicon layer regions 5.

[0049]FIG. 9 illustrates the result after removal of third and fourthinsulation layers 9, 21 at the bottom of trenches 19, 20. Insulationlayers 9, 21 may be removed, for example, by a plasma etching processdirected vertically. In this etching step, fourth insulation layer 21 isremoved not only at the bottom of trenches 17, 18 but also at surface 24of the structure illustrated in FIG. 8. Fourth insulation layer 21 thusremains only on side walls 22 of trenches 17, 18.

[0050] Silicon dioxide layer 15 is also partially removed in thisetching process. Therefore, the silicon dioxide layer 15 may have agreater layer thickness than third insulation layer 9. Since secondinsulation layer 6 is located between trenches 18 and buried polysiliconconductor regions 4, no exposed regions to the buried polysiliconconductor regions 4 are obtained after removing third and fourthinsulation layers 9, 21 at the bottom of trenches 18. Therefore, buriedpolysilicon conductor regions 4 remain completely enclosed by insulationlayer 9.

[0051] After opening third and fourth insulation layers 9, 21 at thebottom of trenches 17, 18, isotropic etching is performed to removesacrificial polysilicon layer regions 5 illustrated in FIG. 10. Anetching medium such as xenon difluoride, chlorine trifluoride or brominetrifluoride is brought to sacrificial polysilicon layer regions 5 byintroducing it through trenches 17. These etching media have a very highselectivity with respect to a non-silicon such as silicon dioxide.

[0052] A cavity 26 having predefined lateral and vertical dimensions isproduced by removing sacrificial polysilicon layer regions 5, withfreely movable structures 25 of the resulting sensor located above thiscavity. Freely movable structures 25, buried polysilicon conductorregions 4 and the other regions of thick polysilicon layer 13 are notetched by the etching media because they are protected by the oxide onall sides.

[0053]FIG. 11 illustrates the layer structure after removing fourthinsulation layer 21 on the side walls of trenches 17, 18, secondinsulation layer 9 on the lower side of freely movable structures 25 aswell as silicon dioxide layer 15 by a vapor etching method using mediacontaining hydrofluoric acid. First insulation layer 2 beneath freelymovable structures 25 may also be removed completely if desired.

[0054]FIG. 11 thus is a cross-sectional view through an exampleembodiment of a sensor element. Various function regions have beenstructured from thick polysilicon layer 13. A terminal region 27, 28,completely surrounded by trenches 18, has been structured out beneathmetal layer 14. These terminal regions 27, 28 are thus completelyinsulated by trenches 18 from the remainder of thick polysilicon layer13. Terminal region 27 is in direct contact with buried polysiliconconductor region 4, so that contact may be established with the otherregions of thick polysilicon layer 13, namely the second neighboringregion on the right side. Terminal region 28 is in direct contact withsilicon substrate 1, thus implementing a substrate contact. Freelymovable structures 25, e.g., parts of interdigital capacitors, arelocated above cavity 26.

[0055] Although the present invention has been described above on thebasis of example embodiments, the present invention is not limited tothese example embodiments but is, instead, modifiable in a variety ofmanners.

[0056] In particular the choice of layer materials described above isonly an example and may be varied as desired. The present invention isalso not limited to acceleration sensors and rotational speed sensors.

What is claimed is:
 1. A micromechanical component including functioncomponents suspended movably above a substrate, comprising: a substrate;a first insulation layer provided above the substrate; a firstmicromechanical function layer including at least one conductor regionprovided above the first insulation layer; a second insulation layerprovided above the conductor region and above the first insulationlayer; a third insulation layer provided above the second insulationlayer; and a second micromechanical function layer provided above thethird insulation layer, the second micromechanical function layerincluding the suspended function components, at least one first trenchand at least one second trench, the first trench extending at least tothe third insulation layer and the second trench extending at least tothe third insulation layer above the conductor region.
 2. Themicromechanical component according to claim 1, wherein the firstmicromechanical function layer and the second micromechanical functionlayer include polysilicon layers.
 3. The micromechanical componentaccording to claim 1, wherein the first, second, and third insulationlayers include oxide layers.
 4. The micromechanical component accordingto claim 1, wherein the second micromechanical function layer includesnonmovable terminal regions connected electrically to the movablysuspended function components.
 5. The micromechanical componentaccording to claim 1, wherein the second and third insulation layersinclude contact holes configured to connect the second micromechanicalfunction layer to the conductor region.
 6. The micromechanical componentaccording to claim 1, wherein the first, second and third insulationlayers include contact holes configured to connect the secondmicromechanical function layer to the substrate.
 7. The micromechanicalcomponent according to claim 1, wherein the micromechanical componentincludes one of an acceleration sensor and a rotational speed sensor. 8.A method of manufacturing a micromechanical component including functioncomponents suspended movably above a substrate, comprising the steps of:(a) preparing the substrate; (b) providing a first insulation layerabove the substrate; (c) providing a first micromechanical functionlayer above the first insulation layer; (d) structuring the firstmicromechanical function layer in a conductor region and a sacrificiallayer region; (e) providing a second insulation layer above thestructure resulting from the structuring step (d); (f) structuring thesecond insulation layer for partially exposing the surface of thesacrificial layer region; (g) providing a third insulation layer abovethe structure resulting from the structuring step (f); (h) providing asecond micromechanical function layer above the structure resulting fromthe providing step (g); (i) structuring the second micromechanicalfunction layer to form at least one first trench and at least one secondtrench, the first trench extending at least to the third insulationlayer above the sacrificial layer region and the second trench extendingat least to the third insulation layer above the conductor region; (j)providing a fourth insulation layer above the structure resulting fromthe structuring step (i); (k) removing the third and fourth insulationlayers at least from the bottom of the first trench; (l) selectivelyetching the sacrificial layer region through the first trench to formthe movably suspended function components in the second micromechanicalfunction layer.
 9. The method according to claim 8, wherein the firstmicromechanical function layer and the second micromechanical functionlayer include polysilicon layers.
 10. The method according to claim 8,wherein the first, second, third, and fourth insulation layers includeoxide layers.
 11. The method according to claim 8, wherein the conductorregion and the sacrificial layer region are provided by localimplantation and subsequent photolithographic structuring.
 12. Themethod according to claim 8, wherein the second and third insulationlayers include contact holes configured to connect the secondmicromechanical function layer to the conductor region.
 13. The methodaccording to claim 8, wherein the first, second and third insulationlayers include contact holes configured to connect the secondmicromechanical function layer to the substrate.
 14. The methodaccording to claim 8, wherein the micromechanical component includes oneof an acceleration sensor and a rotational speed sensor.